Semiconductor device with a low dielectric constant film between lower interconnections

ABSTRACT

A method for fabricating a semiconductor device includes the steps of: forming a plurality of lower interconnections at intervals in a first insulating film; removing a portion of the first insulating film located between the lower interconnections, thereby forming an interconnection-to-interconnection gap; forming a second insulating film over the first insulating film in which the lower interconnections and the interconnection-to-interconnection gap are formed such that an air gap is formed out of the interconnection-to-interconnection gap; and forming, in the second insulating film, a connection portion connected to one of the lower interconnections and an upper interconnection connected to the connection portion. The connection portion is formed to be connected to one of the lower interconnections not adjacent to the air gap.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of U.S. application Ser. No.11/253,568, filed Oct. 20, 2005 now U.S. Pat. No. 7,473,632, claimingpriority of Japanese Patent Application No. 2004-309579, filed Oct. 25,2004, the entire contents of each of which are hereby incorporated byreference.

BACKGROUND OF THE INVENTION

The present invention relates to methods for fabricating semiconductordevices including multilayer interconnection and semiconductor devicesfabricated by the fabrication methods.

Recent remarkable progress of semiconductor processing technology hasenabled significant size reduction and high integration ofinterconnection or devices, so that the performance of ULSI has beenenhanced. With increased integration degree of interconnection, signaldelay in the interconnection has come to determine operation speed ofdevices. In ULSI in 0.25-μm generation or later generation, attempts touse materials having low dielectric constants, SiOC containing organicsubstances or organic materials for interlayer insulating films havebeen made to date. However, these materials have drawbacks inhygroscopicity or heat resistance, and thus it is difficult to establishprocesses using these materials.

To reduce a delay between interconnections, which is a delay having anespecially large influence, a technique for reducing the relativedielectric constant between the interconnections by intentionallyproviding voids (hereinafter, referred to as air gaps) formed by air(∈=1.0) between interconnections in an insulating material has beenproposed. As a method for forming air gaps in a copper interconnectstructure, a method in which an insulating film existing between buriedinterconnections is removed by etching and then another insulating filmis deposited is proposed (see, for example, “A Novel SiO₂-Air Gap low-kCopper Dual Damascene Interconnect” T Micro electronics V. Arnal et.al., p. 71, 2000 Advance Metallization).

Hereinafter, a method for forming air gaps in a copper interconnectstructure will be described with reference to FIGS. 11A through 11D andFIGS. 12A through 12C. FIGS. 11A through 11D and FIGS. 12A through 12Care cross-sectional views of main portions showing a method for formingair gaps in a copper interconnect structure.

First, as shown in FIG. 11A, a first insulating film 10 is depositedover a semiconductor substrate (not shown) on which a semiconductoractive device is formed, and then recesses are formed in the firstinsulating film 10. Subsequently, first barrier metal films 11 areformed on the bottoms and walls of the recesses in the first insulatingfilm 10, and then first interconnections 12 made of copper films areformed so that the recesses are filled therewith.

Next, as shown in FIG. 11B, to prevent peeling of the firstinterconnections 12 and diffusion of copper forming the firstinterconnections 12, a liner insulating film 13 is deposited over thefirst insulating film 10 and the first interconnections 12.

Then, as shown in FIG. 11C, a resist pattern 14 is formed on the linerinsulating film 13 by lithography. The resist pattern 14 has an openingpattern with which only portions of the first insulating film 10 locatedbetween the first interconnections 12 are removed. The resist pattern 14is used to form interconnection-to-interconnection gaps between selectedones of the first interconnections 12 and serves as a mask for exposingonly regions between the selected first interconnections 12.

Thereafter, as shown in FIG. 11D, dry etching is performed using theresist pattern 14 as a mask to etch the liner insulating film 13 and thefirst insulating film 10, thereby forminginterconnection-to-interconnection gaps 15 between the firstinterconnections 12.

Subsequently, as shown in FIG. 12A, a second insulating film 17 isdeposited over the interconnection-to-interconnection gaps 15 betweenthe first interconnections 12 and the liner insulating film 13, therebyforming, between the first interconnections 12, air gaps 16 whose topsproject above the liner insulating film 13. The use of a film having alow coverage rate and poor burying performance as the second insulatingfilm 17 eases formation of the air gaps 16.

Then, as shown in FIG. 12B, etching is performed so that in the secondinsulating film 17, a connecting hole 17 a in which the surface of oneof the first interconnections 12 is exposed is formed and then ainterconnect trench 17 b is formed. In this case, a dual damasceneprocess in which the connecting hole 17 a is formed before theinterconnect trench 17 b is used.

Thereafter, as shown in FIG. 12C, a barrier metal film, a seed film anda plating film are deposited in this order over the second insulatingfilm 17 including the connecting hole 17 a and the interconnect trench17 b, and then excessive portions of the barrier metal film, the seedfilm and the plating film extending off the connecting hole 17 a and theinterconnect trench 17 b are removed by metal-based CMP, thereby forminga via 18 and second interconnections 19. In this manner, a double-layerinterconnect structure made of the first interconnection 12 and thesecond interconnection 19 is formed.

With the foregoing process steps, a semiconductor device including amultilayer interconnect structure in which the air gaps 16 are formedbetween the first interconnections 12 made of copper films isfabricated. The relative dielectric constant of the air gaps 16 made ofair is about ¼ of that of the first insulating film 10. The air gaps 16reduce the capacitance between adjacent ones of first interconnections12. Accordingly, a signal delay between the adjacent firstinterconnections 12 is suppressed, thus implementing a semiconductordevice in which an operation margin is large and malfunction is lesslikely to occur. In addition, conventional interconnection materials canbe used, so that cost reduction is achieved.

SUMMARY OF THE INVENTION

However, it was found that the method for forming air gaps describedabove has the following drawbacks.

First, a problem arises when the resist pattern 14 (see, FIG. 11C)becomes misaligned with the first interconnections 12. Specifically, ifetching is performed using the misaligned resist pattern 14,interconnection-to-interconnection gaps 15 a formed between the firstinterconnections 12 are smaller than theinterconnection-to-interconnection gaps 15 illustrated in FIG. 11D bythe degree corresponding to the misalignment, as shown in FIG. 13A, forexample. In addition to the smaller shape of the gaps between the firstinterconnections 12 as described above, a portion of the linerinsulating film 13 located on the first interconnections 12 is partlyremoved, so that a portion 12 a of the first interconnections 12 isexposed. In this case, during formation of theinterconnection-to-interconnection gaps 15 a between the firstinterconnections 12 by etching, the portion 12 a of the surface of thefirst interconnections 12 made of copper films is oxidized or damaged,for example, resulting in deterioration of the reliability of the firstinterconnections 12.

Another problem arises when misalignment occurs in photolithographyperformed on a structure in which one of the first interconnection 12and the connecting hole 17 a are borderless, i.e., the width of thefirst interconnection 12 is equal to the diameter of the connecting hole17 a. Specifically, as shown in FIG. 13B, if misalignment occurs tocause a shift of the connecting hole 17 a in photolithography, theconnecting hole 17 a becomes continuous with one of the air gaps 16,i.e., penetrates the air gap 16 during formation of the connecting hole17 a. In this case, it is difficult to completely fill the connectinghole 17 a with an interconnection material in a subsequent process step.

It is therefore an object of the present invention to provide a methodfor fabricating a semiconductor device in which interconnection is notdamaged and an air gap and a connecting hole do not become continuous atthe occurrence of misalignment, and also provides a semiconductor devicefabricated by this fabrication method.

A method for fabricating a semiconductor device according to a firstaspect of the present invention includes the steps of: forming aplurality of lower interconnections at intervals in a first insulatingfilm; removing a portion of the first insulating film located betweenthe lower interconnections, thereby forming aninterconnection-to-interconnection gap; forming a second insulating filmover the first insulating film in which the lower interconnections andthe interconnection-to-interconnection gap are formed such that an airgap is formed out of the interconnection-to-interconnection gap; andforming, in the second insulating film, a connection portion connectedto one of the lower interconnections and an upper interconnectionconnected to the connection portion, wherein the connection portion isformed to be connected to one of the lower interconnections not adjacentto the air gap.

In the method according to the first aspect, the connection portion isformed to be connected to one of the lower interconnections not adjacentto the air gap, so that it is possible to prevent penetration of aconnecting hole through the air gap during formation of the connectinghole even at the occurrence of misalignment. In this manner, asemiconductor device including a highly-reliable multilayer interconnectstructure in which interconnection-to-interconnection capacitance isreduced by forming the air gap between the lower interconnections andoccurrence of failures in the connecting hole is prevented isimplemented.

The method according to the first aspect preferably further includes thestep of selectively forming a cap layer on each of the surfaces of thelower interconnections, after the step of forming the lowerinterconnections and before the step of forming theinterconnection-to-interconnection gap.

Then, each of the lower interconnections is covered with the cap layer,so that exposure of the lower interconnections is prevented duringformation of the interconnection-to-interconnection gap or theconnecting hole even at the occurrence of misalignment. Accordingly, thelower interconnections are not damaged, thus implementing ahighly-reliable interconnect structure. In addition, since the cap layeris formed on each of the lower interconnections, a material having smallcapacitance is freely selected as a material for an insulating filmdeposited thereon.

In the method according to the first aspect, the surfaces of the lowerinterconnections are preferably lower than the surface of the firstinsulating film.

Then, the cap layer is easily formed only on the surfaces of the lowerinterconnections whose exposure should be prevented.

In the method according to the first aspect, aninterconnection-to-interconnection space X between the lowerinterconnections sandwiching the air gap preferably satisfies thefollowing relationship:S≦X<3Swhere S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution oflithography.

The provision of the upper limit (3S) of theinterconnection-to-interconnection space described above enablessuppression of decrease of interconnection-to-interconnectioncapacitance. In addition, formation of the air gap becomes easy, so thatan additional step for forming the air gap is not needed, thus reducingthe throughput.

In the method according to the first aspect, the air gap is preferablylocated, from the connection portion, at least at a distance ysatisfying the following relationship:y=S+(L/2)where S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution of lithographyand L is a minimum interconnection width of the lower interconnectionsformed at the minimum resolution of lithography.

Then, even when misalignment occurs, it is possible to prevent an airgap from being formed at both sides of the lower interconnectionconnected to the connection portion. This prevents occurrence offailures in the connecting hole.

Preferably, in the method according to the first aspect, theinterconnection-to-interconnection gap is formed using a resist patternas a mask, and the resist pattern has an opening pattern for exposing aregion which is located between the lower interconnections and in whichthe interconnection-to-interconnection gap is to be formed and a regionexpanded from that region toward the lower interconnections sandwichingthe region by a distance z satisfying the following relationship:z=L/2where L is the minimum interconnection width of the lowerinterconnections formed at the minimum resolution of lithography.

Then, since the resist pattern has the opening pattern for exposing aregion between the lower interconnections where aninterconnection-to-interconnection gap is to be formed, an openingregion does not become small and an interconnection-to-interconnectiongap having a sufficient opening is formed even when misalignment occurs.

The method according to the first aspect preferably further includes thestep of planarizing the surface of the second insulating film by CMPafter the step of forming the second insulating film and before the stepof forming the upper interconnection and the connection portion.

A method for fabricating a semiconductor device according to a secondaspect of the present invention includes the steps of: forming aplurality of lower interconnections at intervals in a first insulatingfilm; removing a portion of the first insulating film located betweenthe lower interconnections, thereby forming aninterconnection-to-interconnection gap; burying alow-dielectric-constant film in the interconnection-to-interconnectiongap; forming a second insulating film over the first insulating film inwhich the lower interconnections and the low-dielectric-constant filmare buried; and forming, in the second insulating film, a connectionportion connected to one of the lower interconnections and an upperinterconnection connected to the connection portion, wherein theconnection portion is formed to be connected to one of the lowerinterconnections not adjacent to the low-dielectric-constant film.

With the method according to the second aspect, the connection portionis formed to be connected to one of the lower interconnections notadjacent to the low-dielectric-constant film, so that thelow-dielectric-constant film is not exposed during formation of theconnecting hole even at the occurrence of misalignment. Accordingly,opening failures (via resist poisoning) such as contamination inside theconnecting hole do not occur. As compared to the method according to thefirst aspect, though the interconnection-to-interconnection capacitanceincreases, a level difference is less likely to be formed in the surfaceof the second insulating film because the low-dielectric-constant filmis buried in the interconnection-to-interconnection gap. Accordingly,the process step of planarizing the level difference can be omitted.Therefore, with the method according to the second aspect, it ispossible to use a low-κ material, whose application to a conventionalsemiconductor fabrication process has been difficult becauseplanarization is difficult because of its properties, for example. Inthis manner, a semiconductor device including a highly-reliablemultilayer interconnect structure in which occurrence of failures in theconnecting hole is prevented is implemented.

The method according to the second aspect preferably further includesthe step of selectively forming a cap layer on each of the surfaces ofthe lower interconnections, after the step of forming the lowerinterconnections and before the step of forming theinterconnection-to-interconnection gap.

Then, each of the lower interconnections is covered with the cap layer,so that exposure of the lower interconnections is prevented duringformation of the interconnection-to-interconnection gap or theconnecting hole even at the occurrence of misalignment. Accordingly, thelower interconnections are not damaged, thus implementing ahighly-reliable interconnect structure. In addition, since the cap layeris formed on each of the lower interconnections, a material having smallcapacitance is freely selected as a material for an insulating filmdeposited thereon.

In the method according to the second aspect, the surfaces of the lowerinterconnections are preferably lower than the surface of the firstinsulating film.

Then, the cap layer is easily formed only on the surfaces of the lowerinterconnections whose exposure should be prevented.

In the method according to the second aspect, aninterconnection-to-interconnection space X between the lowerinterconnections sandwiching the low-dielectric-constant film preferablysatisfies the following relationship:S≦Xwhere S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution oflithography.

In this manner, in the method according to the second aspect, not an airgap but a low-dielectric-constant film is buried in theinterconnection-to-interconnection gap, so that the upper limit of theinterconnection-to-interconnection space does not need to be provided.Accordingly, the flexibility in design is enhanced.

In the method of the second aspect, the low-dielectric-constant film ispreferably located, from the connection portion, at least at a distancey satisfying the following relationship:y=S+(L/2)where S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution of lithographyand L is a minimum interconnection width of the lower interconnectionsformed at the minimum resolution of lithography.

Then, even when misalignment occurs, it is possible to prevent formationof an air gap at both sides of the lower interconnection connected tothe connection portion. Accordingly, occurrence of failures in theconnecting hole is prevented.

Preferably, in the method according to the second aspect, theinterconnection-to-interconnection gap is formed using a resist patternas a mask, and the resist pattern has an opening pattern for exposing aregion which is located between the lower interconnections and in whichthe interconnection-to-interconnection gap is to be formed and a regionexpanded from that region toward the lower interconnections sandwichingthe region by a distance z satisfying the following relationship:z=L/2where L is the minimum interconnection width of the lowerinterconnections formed at the minimum resolution of lithography.

Then, since the resist pattern has the opening pattern with which aregion between the lower interconnections where aninterconnection-to-interconnection gap is to be formed is exposed, anopening region does not become small and aninterconnection-to-interconnection gap having a sufficient opening isformed even when misalignment occurs.

A method for fabricating a semiconductor device according to a thirdaspect of the present invention includes the steps of: forming aplurality of lower interconnections at intervals in a first insulatingfilm; forming a second insulating film over the lower interconnectionsand the first insulating film; removing a portion of the firstinsulating film located between the lower interconnections and a portionof the second insulating film located on the portion of the firstinsulating film, thereby forming an interconnection-to-interconnectiongap; forming a third insulating film over the second insulating film andthe first insulating film in which theinterconnection-to-interconnection gap is formed such that an air gap isformed out of the interconnection-to-interconnection gap; and forming,in the third insulating film, a connection portion connected to one ofthe lower interconnections and an upper interconnection connected to theconnection portion, wherein the connection portion is formed to beconnected to one of the lower interconnections not adjacent to the airgap.

With the method according to the third aspect, the connection portion isformed to be connected to one of the lower interconnections not adjacentto the air gap, so that it is possible to prevent penetration of aconnecting hole through the air gap during formation of the connectinghole even at the occurrence of misalignment. In this manner, asemiconductor device including a highly-reliable multilayer interconnectstructure in which interconnection-to-interconnection capacitance isreduced by forming the air gap between the lower interconnections andoccurrence of failures in the connecting hole is prevented isimplemented.

In the method according to the third aspect, aninterconnection-to-interconnection space X between the lowerinterconnections sandwiching the air gap preferably satisfies thefollowing relationship:S≦X<3Swhere S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution oflithography.

The provision of the upper limit (3S) of theinterconnection-to-interconnection space described above enablessuppression of decrease of interconnection-to-interconnectioncapacitance. In addition, formation of the air gap becomes easy, so thatan additional step for forming the air gap is not needed, thus reducingthe throughput.

In the method according to the third aspect, the air gap is preferablylocated, from the connection portion, at least at a distance ysatisfying the following relationship:y=S+(L/2)where S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution of lithographyand L is a minimum interconnection width of the lower interconnectionsformed at the minimum resolution of lithography.

Then, since the resist pattern has the opening pattern with which aregion between the lower interconnections where aninterconnection-to-interconnection gap is to be formed is exposed, anopening region does not become small and aninterconnection-to-interconnection gap having a sufficient opening isformed even when misalignment occurs.

Preferably, in the method according to the third aspect, the air gap isformed using, as a mask, a first resist pattern having a first openingpattern for exposing a first region. The first opening pattern ispreferably formed so as to have the first region exposed by swelling asecond resist pattern having a second opening pattern for exposing asecond region, which is located between the lower interconnections andwider than the first region.

In this manner, the swelling process makes the first region, which isexposed in the first opening pattern, narrower than the second regionbetween the lower interconnections, so that the first opening pattern isfiner than a pattern formed at the minimum resolution of lithography.Accordingly, even when misalignment occurs, it is possible to preventlower interconnections from being exposed under the removed secondinsulating film, so that the lower interconnections are not damaged.

In the method according to the third aspect, the first resist pattern ispreferably formed by causing a second resist pattern to swell toward themidpoint between the lower interconnections by a distance q satisfyingthe relationship:q=L/3where L is the minimum interconnection width of the lowerinterconnections formed at the minimum resolution of lithography.

Then, even when misalignment occurs, exposure of the lowerinterconnections under the removed second insulating film is preventedwithout fail, thus ensuring prevention of damage on the lowerinterconnections.

The method according to the third aspect preferably further includes thestep of planarizing the surface of the second insulating film by CMPafter the step of forming the second insulating film and before the stepof forming the upper interconnection and the connection portion.

A semiconductor device according to a first aspect of the presentinvention includes: a plurality of lower interconnections formed atintervals in a first insulating film; a second insulating film formedover the lower interconnections and the first insulating film; aconnection portion formed in the second insulating film and connected toone of the lower interconnections; and an upper interconnection formedin the second insulating film and connected to the connection portion,wherein between the lower interconnections, an air gap is formed bycovering, with the second insulating film, aninterconnection-to-interconnection gap formed by removing a portion ofthe first insulating film between the lower interconnections, and theconnection portion is connected to one of the lower interconnections notadjacent to the air gap.

In the semiconductor device according to the first aspect, theconnection portion is connected to one of the lower interconnections notadjacent to the air gap, so that it is possible to prevent penetrationof a connecting hole through the air gap during formation of theconnecting hole even at the occurrence of misalignment. In this manner,a semiconductor device including a highly-reliable multilayerinterconnect structure in which interconnection-to-interconnectioncapacitance is reduced by forming the air gap between the lowerinterconnections and occurrence of failures in the connecting hole isprevented is implemented.

In the device according to the first aspect, aninterconnection-to-interconnection space X between the lowerinterconnections sandwiching the air gap preferably satisfies thefollowing relationship:S≦X<3Swhere S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution oflithography.

The provision of the upper limit (3S) of theinterconnection-to-interconnection space described above enablessuppression of decrease of interconnection-to-interconnectioncapacitance. In addition, formation of the air gap becomes easy, so thatan additional step for forming the air gap is not needed, thus reducingthe throughput.

In the device according to the first aspect, the air gap is preferablylocated, from the connection portion, at least at a distance ysatisfying the following relationship:y=S+(L/2)where S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution of lithographyand L is a minimum interconnection width of the lower interconnectionsformed at the minimum resolution of lithography.

Then, even when misalignment occurs, it is possible to prevent an airgap from being formed at both sides of the lower interconnectionconnected to the connection portion. This prevents occurrence offailures in the connecting hole.

A semiconductor device according to a second aspect of the presentinvention includes: a plurality of lower interconnections formed atintervals in a first insulating film; a second insulating film formedover the lower interconnections and the first insulating film; aconnection portion formed in the second insulating film and connected toone of the lower interconnections; and an upper interconnection formedin the second insulating film and connected to the connection portion,wherein between the lower interconnections, a low-dielectric-constantfilm is formed to fill an interconnection-to-interconnection gap formedby removing a portion of the first insulating film between the lowerinterconnections, and is covered with the second insulating film, andthe connection portion is connected to one of the lower interconnectionsnot adjacent to the low-dielectric-constant film.

In the semiconductor device according to the second aspect, theconnection portion is connected to one of the lower interconnections notadjacent to the low-dielectric-constant film, so that it is possible toprevent exposure of the low-dielectric-constant film during formation ofthe connecting hole even at the occurrence of misalignment. Accordingly,it is possible to prevent opening failures (via resist poisoning) suchas contamination inside the connecting hole from occurring even at theoccurrence of misalignment. As compared to the semiconductor deviceaccording to the first aspect, though theinterconnection-to-interconnection capacitance increases, a leveldifference is less likely to be formed in the surface of the secondinsulating film because the low-dielectric-constant film is buried inthe interconnection-to-interconnection gap. Accordingly, the processstep of planarizing the level difference can be omitted. Therefore, withthe method according to the second aspect, it is possible to use a low-κmaterial, whose application to a conventional semiconductor fabricationprocess has been difficult because planarization is difficult because ofits properties, for example. In this manner, a semiconductor deviceincluding a highly-reliable multilayer interconnect structure in whichoccurrence of failures in the connecting hole is prevented isimplemented.

In the device according to the second aspect, aninterconnection-to-interconnection space X between the lowerinterconnections sandwiching the low-dielectric-constant film preferablysatisfies the following relationship:S≦Xwhere S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution oflithography.

In this manner, in the semiconductor device according to the secondaspect, not an air gap but a low-dielectric-constant film is buried inthe interconnection-to-interconnection gap, so that the upper limit ofthe interconnection-to-interconnection space does not need to beprovided. Accordingly, the flexibility in design is enhanced.

In the device according to the second aspect, thelow-dielectric-constant film is preferably located, from the connectionportion, at least at a distance y satisfying the following relationship:y=S+(L/2)where S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution of lithographyand L is a minimum interconnection width of the lower interconnectionsformed at the minimum resolution of lithography.

Then, even when misalignment occurs, it is possible to prevent formationof an air gap at both sides of the lower interconnection connected tothe connection portion. Accordingly, occurrence of failures in theconnecting hole is prevented.

In the device according to the first or second aspect, a cap layer ispreferably formed between each of the surfaces of the lowerinterconnections and the second insulating film.

Then, each of the lower interconnections is covered with the cap layer,so that exposure of the lower interconnections is prevented duringformation of the interconnection-to-interconnection gap or theconnecting hole even at the occurrence of misalignment. Accordingly, thelower interconnections are not damaged, thus implementing ahighly-reliable interconnect structure. In addition, since the cap layeris formed on each of the lower interconnections, a material having smallcapacitance is freely selected as a material for an insulating filmdeposited thereon.

In the device according to the first or second aspect, it is preferablethat the cap layer has a width larger than the interconnection width ofthe lower interconnections and has an eave for an associated one of thelower interconnections.

Then, during the second insulating film, the second insulating filmgrowing on the eaves covers the interconnection-to-interconnection gap,so that a large air gap is formed. Accordingly,interconnection-to-interconnection capacitance is further reduced.

In the semiconductor device according to the first or second aspect, thesurfaces of the lower interconnections are preferably lower than thesurface of the first insulating film.

Then, the cap layer is easily formed only on the surfaces of the lowerinterconnections whose exposure should be prevented.

A semiconductor device according to a third aspect of the presentinvention includes: a plurality of lower interconnections formed atintervals in a first insulating film; a second insulating film formedover the lower interconnections and the first insulating film; a thirdinsulating film formed on the second insulating film; a connectionportion formed in the third insulating film and connected to one of thelower interconnections; and an upper interconnection formed in the thirdinsulating film and connected to the connection portion, wherein betweenthe lower interconnections, an air gap is formed by covering, with thethird insulating film, an interconnection-to-interconnection gap formedby removing a portion of the first insulating film between the lowerinterconnections and a portion of the second insulating film on theportion of the first insulating film, and the connection portion isconnected to one of the lower interconnections not adjacent to the airgap.

In the semiconductor device according to the third aspect, theconnection portion is connected to one of the lower interconnections notadjacent to the air gap, so that it is possible to prevent penetrationof a connecting hole through the air gap during formation of theconnecting hole even at the occurrence of misalignment. In this manner,a semiconductor device including a highly-reliable multilayerinterconnect structure in which interconnection-to-interconnectioncapacitance is reduced by forming the air gap between the lowerinterconnections and occurrence of failures in the connecting hole isprevented is implemented.

In the device according to the third aspect, aninterconnection-to-interconnection space X between the lowerinterconnections sandwiching the air gap preferably satisfies thefollowing relationship:S≦X<3Swhere S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution oflithography.

The provision of the upper limit (3S) of theinterconnection-to-interconnection space described above enablessuppression of decrease of interconnection-to-interconnectioncapacitance. In addition, formation of the air gap becomes easy, so thatan additional step for forming the air gap is not needed, thus reducingthe throughput.

In the device according to the third aspect, the air gap is preferablylocated, from the connection portion, at least at a distance ysatisfying the following relationship:y=S+(L/2)where S is a minimum interconnection-to-interconnection space betweenthe lower interconnections formed at a minimum resolution of lithographyand L is a minimum interconnection width of the lower interconnectionsformed at the minimum resolution of lithography.

Then, even when misalignment occurs, it is possible to prevent an airgap from being formed at both sides of the lower interconnectionconnected to the connection portion. This prevents occurrence offailures in the connecting hole.

In the device according to the third aspect, the second insulating filmpreferably has an eave projecting from the edge of the each of the lowerinterconnections adjacent to the air gap.

Then, during formation of the third insulating film, the thirdinsulating film growing on the eaves covers theinterconnection-to-interconnection gap, so that a large air gap isformed. Accordingly, interconnection-to-interconnection capacitance isfurther reduced.

In the device according to the third aspect, the eave of the secondinsulating film preferably projects from the edge of the each of thelower interconnections by a distance of L/3 where L is a minimuminterconnection width of the lower interconnections formed at a minimumresolution of lithography.

Then, the second insulating film has an eave projecting by the distanceof L/3, so that the lower interconnections are not exposed duringformation of an interconnection-to-interconnection gap even whenmisalignment occurs. Accordingly, the lower interconnections are notdamaged.

In the device according to the first, second or third aspect, each ofthe lower interconnections and the upper interconnection is preferablymade of a metal mainly containing Cu.

In the device according to the first or second aspect, each of the firstand second insulating films is preferably made of SiO₂, FSG, SiOC or anorganic polymer.

In the device according to the third aspect, each of the first and thirdinsulating films is preferably made of SiO₂, FSG, SiOC or an organicpolymer.

In the device according to the first or second aspect, the cap layer ispreferably made of one or more materials selected from the groupconsisting of Ta, TaN, Ti, TiN, W, WCoP, CoB and NiMoP.

In the device according to the first, second or third aspect, the bottomof the interconnection-to-interconnection gap is preferably located at aposition deeper than the bottom of one of the lower interconnectionsadjacent to the interconnection-to-interconnection gap by about ⅓ of theinterconnection width of the lower interconnection.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1E are cross-sectional views of main portions showing amethod for fabricating a semiconductor device according to a firstembodiment of the present invention.

FIGS. 2A through 2D are cross-sectional views of main portions showingthe method for fabricating a semiconductor device according to the firstembodiment.

FIG. 3 is a plan view illustrating a mask layout of a resist patternaccording to the first embodiment.

FIG. 4 is a graph showing a relationship between theinterconnection-to-interconnection space and theinterconnection-to-interconnection capacitance in the first embodiment.

FIGS. 5A through 5D are cross-sectional views of main portions showing amethod for fabricating a semiconductor device according to a secondembodiment of the present invention.

FIGS. 6A through 6D are cross-sectional views of main portions showingthe method for fabricating a semiconductor device according to thesecond embodiment.

FIGS. 7A through 7D are cross-sectional views of main portions showing amethod for fabricating a semiconductor device according to a thirdembodiment of the present invention.

FIGS. 8A through 8E are cross-sectional views of main portions showing amethod for fabricating a semiconductor device according to a fourthembodiment of the present invention.

FIGS. 9A through 9E are cross-sectional views of main portions showingthe method for fabricating a semiconductor device according to thefourth embodiment.

FIG. 10 is a plan view illustrating a mask layout of a resist patternaccording to the fourth embodiment.

FIGS. 11A through 11D are cross-sectional views of main portions showinga method for forming air gaps in a copper interconnect structureaccording to a conventional example.

FIGS. 12A through 12C are cross-sectional views of main portions showingthe method for forming air gaps in a copper interconnect structureaccording to the conventional example.

FIGS. 13A and 13B are cross-sectional views of main portions showing acopper interconnect structure including air gaps at the occurrence ofmisalignment, which is a problem to be solved by the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

Embodiment 1

A semiconductor device and a method for fabricating the device accordingto a first embodiment of the present invention will be described withreference to FIGS. 1A through 1E and FIGS. 2A through 2D.

First, as shown in FIG. 1A, a first insulating film 101 is depositedover a semiconductor substrate (not shown) in which a semiconductoractive device is formed, and then recesses to be interconnect trenchesare formed in the first insulating film 101 by lithography and dryetching. Subsequently, a barrier metal film is deposited over the firstinsulating film 101 including the bottoms and walls of the recesses, andthen a metal film made of, for example, a copper film is deposited sothat the recesses are filled therewith. Thereafter, portions of thebarrier metal film and the metal film extending off the recesses in thefirst insulating film 101 are removed by chemical mechanical polishing(CMP), thereby forming first barrier metal films 102 and firstinterconnections 103, respectively. In this embodiment, the firstinsulating film 101 is a silicon dioxide (SiO₂) film. Alternatively, anyof other insulating materials for use in semiconductor processing, e.g.,FSG or a low-κ material, may be used. The first barrier metal films 102are generally made of tantalum (Ta), tantalum nitride (TaN) or amultilayer structure of these materials.

As shown in FIG. 1A, recesses 103 a are formed such that the surfaces ofthe first interconnections 103 are lower than the surface of the firstinsulating film 101. Such a structure is formed by excessivelyperforming removal and polishing with the conditions for the CMP processdescribed above adjusted.

Next, as shown in FIG. 1B, a cap metal film 104 is deposited over thefirst insulating film 101 and the first interconnections 103.

As the first barrier metal films 102, the cap metal film 104 may be madeof tantalum (Ta), tantalum nitride (TaN) or a multilayer structure ofthese materials. The cap metal film 104 is deposited to have a thicknesslarger than that of the recesses 103 a.

Then, as shown in FIG. 1C, portions of the cap metal film 104 remainingon the first insulating film 101 are removed by CMP such that theresultant cap metal films 104 have a thickness equal to that of therecesses 103 a.

Thereafter, as shown in FIG. 1D, a resist pattern 105 for forming an airgap is formed on the first insulating film 101 and the cap metal films104. The resist pattern 105 is a pattern for removing a portion of thefirst insulating film 101 located between selected ones of the firstinterconnections 103. Specifically, the resist pattern 105 has anopening pattern for exposing a portion 101 a of the first insulatingfilm 101 between the selected first interconnections 103 and alsoexposing portions 105 a on the upper faces of the cap metal films 104sandwiching the portion 101 a. In this manner, the resist pattern 105has the opening pattern in which a separation for exposing the portions105 a as well as the portion 101 a is provided in consideration ofoccurrence of misalignment of the resist pattern 105. The length of theseparation corresponding to the portions 105 a is half of the minimuminterconnection width of an interconnection formed at the minimumresolution of lithography. This will be specifically described later.

Then, as shown in FIG. 1E, etching is performed using the resist pattern105 as a mask to remove a portion of the first insulating film 101located between the first interconnections 103, thereby forming aninterconnection-to-interconnection gap 106. In this case, the cap metalfilms 104 adjacent to a region in which theinterconnection-to-interconnection gap 106 is to be formed is not etchedat all under the etching conditions, so that theinterconnection-to-interconnection gap 106 is easily formed withself-alignment. In addition, as the etching for forming theinterconnection-to-interconnection gap 106, isotropic etching using asmall amount of ion components is performed, so that the edges of thefirst interconnections 103 are not rounded. In forming theinterconnection-to-interconnection gap 106, the amount of a recessedportion 106 a at the bottom of the interconnection-to-interconnectiongap 106 is adjusted, so that it is possible to finely adjust theinterconnection-to-interconnection capacitance. In this case, the depthof the recessed portion 106 a is set at about ⅓ of the interconnectionwidth of the first interconnections 103. This value is determined inconsideration of the necessity of preventing an air gap, which will beformed in a subsequent process step, from reaching the bottom of anupper-level interconnection, which will be also formed in a subsequentprocess step, and the necessity of adjusting theinterconnection-to-interconnection capacitance. However, the presentinvention is not limited to this value, and the depth of the recessedportion 106 a may be set at another value.

An advantage in this process step is that the first interconnections 103made of copper films are not damaged at all during the etching forforming the interconnection-to-interconnection gap 106. This is becausethe first interconnections 103 are covered with the cap metal films 104and, therefore, the surfaces of the first interconnections 103 are notexposed during the etching. This aspect makes this embodiment differfrom the conventional example in which etching damage occurs in thecopper interconnect structure. Since the surfaces of the firstinterconnections 103 are covered with the cap metal films 104, the firstinterconnections 103 are not damaged even at the occurrence of a shiftof the resist pattern 105 due to misalignment, in the same manner.Accordingly, it is possible to prevent etching damage on the surfaces ofthe first interconnections 103, so that the structure of this embodimentis very useful for enhancing the yield and reliability of the firstinterconnections 103 and also effective in preventing peeling ofinsulating films formed on the first interconnections 103 and also inpreventing occurrence of cracks. In addition, when the firstinterconnections 103 are subjected to etching damage, contamination ofetching apparatus caused by scattering of copper in the apparatus is aproblem. However, the process step described above prevents etchingdamage on the first interconnections 103, so that the problem ofcontamination of the etching apparatus does not occur.

Then, as shown in FIG. 2A, a second insulating film 107 is depositedover the first insulating film 101, the cap metal films 104 and theinterconnection-to-interconnection gap 106, thereby forming an air gap108 out of the interconnection-to-interconnection gap 106 between thefirst interconnections 103. As a material for the second insulating film107, a material exhibiting poor step coverage is preferably used becausethe purpose in using this material is to form the air gap 108 out of theinterconnection-to-interconnection gap 106. In this embodiment, SiO₂ isused as a material for the second insulating film 107. Alternatively,any of other insulating materials for use in semiconductor processing,e.g., FSG or a low-κ material, may be used. In the conventional example,an insulating film deposited after formation of the firstinterconnections 103 is made of SiN or SiC(N) forming an insulating filmpreventing diffusion of copper. On the other hand, in this embodiment,since the cap metal films 104 are formed on the surfaces of the firstinterconnections 103, prevention of copper diffusion does not need to betaken into consideration. Accordingly, a material having smallcapacitance is freely selected as a material for the second insulatingfilm 107. In addition, a level difference 107 a is formed in a surfaceportion of the second insulating film 107 above the air gap 108.

Thereafter, as shown in FIG. 2B, the surface of the second insulatingfilm 107 including the level difference 107 a is planarized by CMP.

Subsequently, as shown in FIG. 2C, lithography and dry etching areperformed to form, in the second insulating film 107, a connecting hole107 b in which one of the cap metal films 104 is exposed andinterconnect trenches 107 c. As in the conventional example, theconnecting hole 107 b and the interconnect trenches 107 c are formed bya dual damascene process.

FIG. 2C shows a state in which the connecting hole 107 b slightly shiftsfrom an associated one of the first interconnections 103 at theoccurrence of misalignment. Even in such a case where misalignmentoccurs, failures such as penetration of the connecting hole 107 bthrough the air gap 108 do not occur at all because no air gap 108 isformed at both sides of the first interconnection 103 to which theconnecting hole 107 b is connected.

In addition, since the surfaces of the first interconnections 103 arecovered with the cap metal films 104, the first interconnections 103 arenot exposed during etching for forming the connecting hole 107 b.Accordingly, the same advantages as those in the etching shown in FIG.1E are obtained.

Thereafter, as shown in FIG. 2D, a metal film is buried in theconnecting hole 107 b and the interconnect trenches 107 c, and thenportions of the metal film extending off the connecting hole 107 b andthe interconnect trenches 107 c are removed by CMP, thereby forming avia 109 and second interconnections 110.

Now, a mask layout 3A for forming the resist pattern 105 for use in theprocess step shown in FIG. 1D will be specifically described withreference to FIG. 3. FIG. 3 is a schematic plan view for describing anexample of a mask layout of the resist pattern shown in FIG. 1D. FIG. 1Dcorresponds to a cross-sectional view taken along the line Id-Id shownin FIG. 3.

In FIG. 3, the interconnection width (minimum interconnection width) ofthe first interconnections 103 formed at the minimum resolution oflithography is represented as L and the distance between the firstinterconnections 103 (minimum interconnection-to-interconnection space)formed at the minimum resolution of lithography is represented as S. InFIG. 3, the positions of the connecting holes 107 b (see, FIG. 2C) arealso shown.

As illustrated in FIG. 3, the mask layout 3A of the resist pattern 105includes: a mask region 3 a (provided with dots) covering, with aresist, regions the connecting holes 107 b and a region in which thefirst interconnections 103 are widely spaced; and an opening region 3 b(provided with no dots) for exposing the other region.

The mask layout 3A is formed by automatic design in which a regionhaving an interconnection-to-interconnection space wider than or equalto S and narrower than 3S is automatically detected and an openingregion is formed in the detected region. In this case, the openingregion is expanded by a distance of L/2 toward the firstinterconnections 103 adjacent to a portion where an opening is to beformed. In addition, in this case, in a case where the edge of theexpanded opening region overlaps with the edges of another expandedopening region, the overlapping edges are merged to form one figure,i.e., one opening region. With respect to the regions for the connectingholes 107 b, a mask region is formed so as to mask portions eachexpanded by S+(L/2) from an associated one of the regions where theconnecting holes 107 b are to be formed, and then this mask region issubtracted from the previously-formed opening region. In this manner,the mask layout 3A including the mask region 3 a and the opening region3 b for forming the resist pattern 105 shown in FIG. 1D is designed.

Now, first, a reason for detecting a region having aninterconnection-to-interconnection distance (space) larger than or equalto S and less than 3S, i.e., a reason for setting the upper limit of theinterconnection-to-interconnection space at a value less than 3S, willbe specifically described with reference to FIG. 4.

FIG. 4 is a graph showing a relationship between theinterconnection-to-interconnection space and theinterconnection-to-interconnection capacitance. In FIG. 4, the ordinaterepresents the interconnection-to-interconnection capacitance value(pF/mm) and the abscissa represents theinterconnection-to-interconnection space (μm). FIG. 4 is provided withsupplementary views 4 a, 4 b, 4 c and 4 d schematically illustratingrespective interconnect structures. The supplementary views 4 a and 4 billustrate general cross-sectional structures of interconnection. Thesupplementary views 4 c and 4 d illustrate general cross-sectionalstructures in which air gaps are formed between interconnections.

First, as shown in FIG. 4, in a structure in which no air gaps areformed between interconnections, the parasitic capacitance value betweeninterconnections increases as the interconnection-to-interconnectionspace decreases from the structure illustrated in the supplementary view4 b to the structure illustrated in the supplementary view 4 a, forexample. The increase of the parasitic capacitance value is pronouncedat an interconnection-to-interconnection space of 0.5 μm or less, and atan interconnection-to-interconnection space of approximately 0.25 μm,the parasitic capacitance value is approximately twice as large as thatat an interconnection-to-interconnection space of 1 μm.

On the other hand, as shown in FIG. 4, in a structure in which air gapsare formed between interconnections, e.g., in the structure illustratedin the supplementary view 4 c, though theinterconnection-to-interconnection space is small, the parasiticcapacitance value is suppressed to substantially the same value as thatin a structure in which no air gaps are formed, e.g., the structureillustrated in the supplementary view 4 b. In particular, as compared toa structure in which no air gaps are formed, e.g., the structureillustrated in the supplementary view 4 a, about 60% reduction of theparasitic capacitance value between interconnections is achieved.

In a structure in which the interconnection-to-interconnection space iswide, e.g., the interconnect structure including an air gap asillustrated in the supplementary view 4 d, it is found that theparasitic capacitance value between interconnections does not largelydiffer from that in an interconnect structure in which no air gaps areformed as illustrated in, for example, the supplementary view 4 b.

That is, the advantage obtained by forming air gaps betweeninterconnections is significant in reduction of parasitic capacitancebetween interconnections when the interconnection-to-interconnectionspace is narrow, but is not significant when theinterconnection-to-interconnection space is wide. Specifically, asillustrated in the supplementary views 4 a and 4 c, for example, whenthe interconnection-to-interconnection space is three or more times aswide as the minimum interconnection-to-interconnection space, e.g., isabout 1 μm, an air gap does not need to be formed betweeninterconnections, and a structure in which no air gaps are formedbetween interconnections, as illustrated in, for example, thesupplementary view 4 b is more preferable.

With respect to an interconnection-to-interconnection space suitable forformation of air gaps, a factor from the viewpoint of the fabricationprocess needs to be taken into consideration. In general, as theinterconnection-to-interconnection space becomes narrower, it becomesmore difficult to fill a gap between interconnections and voids are morelikely to occur, so that air gaps are easily formed. On the other hand,as the interconnection-to-interconnection space becomes wider, itbecomes easier to fill a gap between interconnections, so that formationof air gaps becomes inevitably difficult. Accordingly, an additionalprocess for forming air gaps is needed, thus reducing the throughput.When the interconnection-to-interconnection space is 3S or more, therearises a problem in which a large level difference is formed in thesurface of an insulating film deposited after formation of an air gapbetween interconnections so that subsequent planarization is difficult.

Accordingly, based on the distance between interconnections, i.e., theminimum interconnection-to-interconnection space called theinterconnection-to-interconnection space, a structure including air gapsis preferably employed for an interconnection-to-interconnection spaceless than three times as wide as the minimuminterconnection-to-interconnection space, whereas a structure includingno air gaps is preferably employed for aninterconnection-to-interconnection space three or more times as wide asthe minimum interconnection-to-interconnection space.

Then, a reason for expanding the opening region 3 b by a half of theminimum interconnection width L is associated with misalignment.Specifically, it is found that the misalignment occurs by the degree ofabout ⅓ of the minimum interconnection width L. In view of this, if theopening region 3 b is expanded by L/2, it is possible to forminterconnection-to-interconnection gaps 106 not only in part of regionsbetween the first interconnections 103 but also in almost all theregions even in a case where misalignment occurs laterally. Accordingly,the interconnection-to-interconnection gaps 106 do not become small, andlarge air gaps 108 are formed.

A reason for masking portions expanded from regions where connectingholes 107 b are formed by the distance S+(L/2) is also associated withthe misalignment described above, and is to prevent formation of anopening region at both sides of the first interconnections 103 connectedto the connecting holes 107 b. Specifically, a resist pattern maskingportions expanded from the formation regions of the connecting holes 107b by the distance of S+(L/2) prevents formation of an opening region atboth sides of the first interconnections 103 connected to the connectingholes 107 b even when misalignment occurs laterally. Accordingly, evenwhen the connecting holes 107 b shift laterally because of misalignment,a problem of making a connection hole and aninterconnection-to-interconnection gap continuous as described above isprevented.

The use of the mask layout 3A designed as described above allowsformation of a resist pattern 105 with which an air gap 108 is formed inan interconnection-to-interconnection space equal to or wider than S andnarrower than 3S and no air gap 108 is formed at both sides of the firstinterconnection 103 connected to the second interconnection 110 throughthe via 109.

Embodiment 2

A semiconductor device and a method for fabricating the device accordingto a second embodiment of the present invention will be described withreference to FIGS. 5A through 5D and FIGS. 6A through 6D.

First, as shown in FIG. 5A, a first insulating film 201 is depositedover a semiconductor substrate (not shown) in which a semiconductoractive device is formed. Subsequently, recesses to be interconnecttrenches are formed in the first insulating film 201 by lithography anddry etching. Then, a barrier metal film is deposited over the firstinsulating film 201 including the bottoms and walls of the recesses, andthen a metal film made of, for example, a copper film is deposited sothat the recesses are filled therewith. Thereafter, portions of thebarrier metal film and the metal film extending off the recesses in thefirst insulating film 201 are removed by chemical mechanical polishing(CMP), thereby forming first barrier metal films 202 and firstinterconnections 203, respectively.

In this embodiment, the first insulating film 201 is a silicon dioxide(SiO₂) film. Alternatively, any of other insulating materials for use insemiconductor processing, e.g., FSG or a low-κ material, may be used.The first barrier metal films 202 are generally made of tantalum (Ta),tantalum nitride (TaN) or a multilayer structure of these materials.

Next, as shown in FIG. 5B, cap metal films 204 made of a CoWP film areselectively deposited only over the upper faces of the firstinterconnections 203. In this embodiment, a CoWP film is used as the capmetal films 204. Alternatively, as the cap metal films 204 that can beselectively deposited, a CoB film or a NiMoP film, which can beselectively deposited only over metal interconnections may be used usingelectrodeless plating, as introduced in IITC2004 p. 75 “High ReliabilityCu Interconnection Utilizing a Low Contamination CoWP layer” or otherliteratures. These selectively-deposited cap metal films 204 growisotropically with respect to the surfaces of the first interconnections203. Accordingly, each of the cap metal films 204 has features of beingwider than an associated one of the first interconnections 203 andhaving eaves 204 a projecting from the edges of the firstinterconnection 203.

Thereafter, as shown in FIG. 5C, a resist pattern 205 for forming an airgap is formed on the first insulating film 201 and the cap metal films204. The resist pattern 205 is a pattern for removing a portion of thefirst insulating film 201 located between selected ones of the firstinterconnections 203. Specifically, the resist pattern 205 has anopening pattern for exposing a portion 201 a of the first insulatingfilm 201 between the selected first interconnections 203 and alsoexposing portions 205 a of the upper faces of the cap metal films 204adjacent to the portion 201 a. In this manner, the resist pattern 205has the opening pattern in which a separation for exposing the portions205 a as well as the portion 201 a is provided in consideration ofoccurrence of misalignment of the resist pattern 205. The length of theseparation corresponding to the portions 205 a is half of the minimumwidth of an interconnection formed at the minimum resolution oflithography. This will be specifically described later.

Then, as shown in FIG. 5D, etching is performed using the resist pattern205 as a mask to remove a portion of the first insulating film 201located between the first interconnections 203, thereby forming aninterconnection-to-interconnection gap 206. In this case, the cap metalfilms 204 adjacent to a region in which theinterconnection-to-interconnection gap 206 is to be formed is not etchedat all under the etching conditions, so that theinterconnection-to-interconnection gap 206 is easily formed in aself-aligned manner. In addition, as the etching for forming theinterconnection-to-interconnection gap 206, isotropic etching using asmall amount of ion components is performed, so that the eaves 204 a ofthe cap metal films 204 remain without change to prevent rounding of theedges of the first interconnections 203. In forming theinterconnection-to-interconnection gap 206, the amount of a recessedportion 206 a at the bottom of the interconnection-to-interconnectiongap 206 is adjusted, so that it is possible to finely adjustinterconnection-to-interconnection capacitance. In this case, the depthof the recessed portion 206 a is set at about ⅓ of the interconnectionwidth of the first interconnections 203. This value is determined inconsideration of the necessity of preventing an air gap, which will beformed in a subsequent process step, from reaching the bottom of anupper-level interconnection, which will be also formed in a subsequentprocess step, and of the necessity of adjusting theinterconnection-to-interconnection capacitance. However, the presentinvention is not limited to this value, and the depth of the recessedportion 206 a may be set at another value.

An advantage obtained by the foregoing process steps is that the firstinterconnections 203 made of copper films are not damaged at all duringetching for forming the interconnection-to-interconnection gap 206. Thisis because the first interconnections 203 are covered with the cap metalfilms 204 so that the upper faces of the first interconnections 203 arenot exposed during the etching. This structure makes this embodimentdiffer from the conventional example in which copper interconnectionsare subjected to etching damage. In addition, since the upper faces ofthe first interconnections 203 are covered with the cap metal films 204,the first interconnections 203 are not damaged either, even when theposition of the resist pattern 205 shifts because of misalignment. Inthis manner, it is possible to prevent etching damage on the surfaces ofthe first interconnections 203, so that this embodiment is very usefulfor enhancing the yield and reliability of the first interconnections203 and, in addition, for preventing peeling of insulating films formedon the first interconnections 203 and occurrence of cracks. Furthermore,if the first interconnections 203 are subjected to etching damage, therearises the problem of contamination of etching apparatus caused byscattering of copper into the apparatus. In the foregoing process steps,however, the first interconnections 203 are free from etching damage, sothat the problem of contamination of etching apparatus does not arise.

Then, as shown in FIG. 6A, a second insulating film 207 is depositedover the first insulating film 201, the cap metal films 204 and theinterconnection-to-interconnection gap 206, thereby forming an air gap208 out of the interconnection-to-interconnection gap 206 between thefirst interconnections 203. As a material for the second insulating film207, a material exhibiting poor step coverage is preferably used becausethe purpose in using this material is to form the air gap 208 out of theinterconnection-to-interconnection gap 206. In this embodiment, SiO₂ isused for the second insulating film 207. Alternatively, any of otherinsulating materials for use in semiconductor processing, e.g., FSG or alow-κ material, may be used. In the conventional example, an insulatingfilm deposited after formation of the first interconnections 203 is madeof, for example SiN or SiC(N) forming an insulating film preventingdiffusion of copper. On the other hand, in this embodiment, since thecap metal films 204 are formed on the surfaces of the firstinterconnections 203, prevention of copper diffusion does not need to betaken into consideration. Accordingly, a material having smallcapacitance is freely selected as a material for the second insulatingfilm 207. In addition, a level difference 207 a is formed in a surfaceportion of the second insulating film 207 located above the air gap 208.

Thereafter, as shown in FIG. 6B, the surface of the second insulatingfilm 207 including the level difference 207 a is planarized by CMP.

Subsequently, as shown in FIG. 6C, lithography and dry etching areperformed to form, in the second insulating film 207, a connecting hole207 b in which one of the cap metal films 204 is exposed andinterconnect trenches 207 c. As in the conventional example, theconnecting hole 207 b and the interconnect trenches 207 c are formed bya dual damascene process.

FIG. 6C shows a state in which the connecting hole 207 b slightly shiftsfrom an associated one of the first interconnections 203 at theoccurrence of misalignment. Even in such a case where misalignmentoccurs, failures such as penetration of the connecting hole 207 bthrough the air gap 208 do not occur at all because no air gap 208 isformed at both sides of the first interconnection 203 to which theconnecting hole 207 b is connected.

In addition, since the surfaces of the first interconnections 203 arecovered with the cap metal films 204, the first interconnections 203 arenot exposed during etching for forming the connecting hole 207 b.Accordingly, the same advantages as those in the etching shown in FIG.5D are obtained.

Thereafter, as shown in FIG. 6D, a metal film is buried in theconnecting hole 207 b and the interconnect trenches 207 c, and thenportions of the metal film extending off the connecting hole 207 b andthe interconnect trenches 207 c are removed by CMP, thereby forming avia 209 and second interconnections 210.

A mask layout for forming the resist pattern used in the process stepshown in FIG. 5C is the same as that illustrated in FIG. 3 in the firstembodiment, and description thereof is herein omitted.

The second embodiment has the same advantages as those of the firstembodiment, but is different from the first embodiment in the followingaspects. That is, since the cap metal films 204 are provided with theeaves 204 a, the second insulating film 207 formed on the eaves 204 acovers the interconnection-to-interconnection gap 206 during depositionof the second insulating film 207 at a subsequent process step so thatthe air gap 208 having a larger size is formed. In addition, in thefirst embodiment, in the case of forming the cap metal films 204 on thefirst interconnections 203 having a large interconnection width, it isdifficult to form the cap metal films 204 because of characteristics ofCMP. On the other hand, the second embodiment has a great feature inwhich selective growth of metal is utilized to enable uniform depositionof the cap metal films 204 independently of the interconnection width ofinterconnections underlying the cap metal films 204.

Embodiment 3

A semiconductor device and a method for fabricating the device accordingto a third embodiment of the present invention will be described withreference to FIGS. 5A through 5D and FIGS. 7A through 7D.

First, as in the description with reference to FIGS. 5A through 5D,barrier metal films 302 and first interconnections 303 are formed inthis order in recesses formed in a first insulating film 301.Thereafter, cap metal films 304 provided with eaves are formed, and thenan interconnection-to-interconnection gap 306 is formed between thefirst interconnections 303.

Next, as shown in FIG. 7A, a low-κ material 311 is deposited over thefirst insulating film 301, the cap metal films 304 and theinterconnection-to-interconnection gap 306. A feature of this embodimentis that a low-κ material having excellent flowability and formed bycoating and baking is used as the low-κ material 311. The low-κ materialis poured into the interconnection-to-interconnection gap 306 forcoating. In this embodiment, a low-κ material mainly containing anorganic material is used, but an inorganic coating material or a porousmaterial may also be used.

Then, as shown in FIG. 7B, after the low-κ material 311 has beensintered, portions of the low-κ material 311 extending off theinterconnection-to-interconnection gap 306 is removed by, for exampleCMP or wet etching, thereby leaving the low-κ material 311 only insidethe interconnection-to-interconnection gap 306. Subsequently, a secondinsulating film 307 is deposited over the first insulating film 301, thecap metal films 304 and the low-κ material 311. In the conventionalexample, an insulating film deposited after formation of the firstinterconnections 303 is made of SiN or SiC(N), for example, serving asan insulating film preventing diffusion of copper. On the other hand, inthis embodiment, since the cap metal films 304 are formed on thesurfaces of the first interconnections 303, prevention of Cu diffusiondoes not need to be taken into consideration. Accordingly, a materialhaving small capacitance is freely selected as a material for the secondinsulating film 307. In the first and second embodiments, formation ofan air gap causes a level difference (107 a, 207 a) in the surface ofthe second insulating film (107, 207). On the other hand, in thisembodiment, the low-κ material 311 is formed in theinterconnection-to-interconnection gap 306 without formation of an airgap, so that no level difference is formed in the surface of the secondinsulating film 307 after deposition of the second insulating film 307.Accordingly, unlike the first and second embodiments, planarizationusing CMP is unnecessary after deposition of the second insulating film307.

Thereafter, as shown in FIG. 7C, a connecting hole 307 b in which one ofthe cap metal films 304 is exposed and interconnect trenches 307 c areformed in the second insulating film 307 using lithography and dryetching. As in the conventional example, the connecting hole 307 b andthe interconnection trenches 307 c are formed by a dual damasceneprocess.

FIG. 7C shows a state in which the connecting hole 307 b slightly shiftsfrom an associated one of the first interconnections 303 at theoccurrence of misalignment. Even in such a case where misalignmentoccurs, the low-κ material 311 is not exposed during formation of theconnecting hole 307 b and the interconnection trenches 307 c because thelow-κ material 311 does not exist at both sides of the firstinterconnection 303 to which the connecting hole 307 b is connected.Accordingly, it is possible to prevent opening failures (via resistpoisoning) such as contamination of the connecting hole 307 b, causedduring formation of a resist pattern for forming the interconnectiontrenches 307 c after formation of the connecting hole 307 b.

In addition, since the surfaces of the first interconnections 303 arecovered with the cap metal films 304, the first interconnections 303 arenot exposed during etching for forming the connecting hole 307 b.Accordingly, the same advantages as those obtained in the etching forforming the interconnection-to-interconnection gap 306 are obtained.

Then, as shown in FIG. 7D, metal films are buried in the connecting hole307 b and the interconnect trenches 307 c, and then portions of themetal films extending off the connecting hole 307 b and the interconnecttrenches 307 c are removed by CMP, thereby forming a via 309 and secondinterconnections 310.

The resist pattern used for forming theinterconnection-to-interconnection gap 306 is the same as that for themask layout described in the first embodiment, and description of thesame part thereof is herein omitted. In the first and secondembodiments, as shown in FIG. 3, a region where aninterconnection-to-interconnection space is wider than or equal to S andnarrower than 3S is detected by automatic design. On the other hand, inthis third embodiment, the upper limit of 3S is not needed for theinterconnection-to-interconnection space. This is because not an air gapbut the low-κ material 311 is buried in theinterconnection-to-interconnection gap 306, so that decrease of theinterconnection-to-interconnection capacitance according to aninterconnection-to-interconnection space do not need to be taken intoconsideration and no level difference is formed in the surface of thesubsequently-deposited second insulating film 307, thus eliminating thenecessity of providing the upper limit of theinterconnection-to-interconnection space in consideration of a leveldifference formed in the surface, unlike the first and secondembodiments.

Unlike the first and second embodiments in which an air gap is formedout of the interconnection-to-interconnection gap 306, the thirdembodiment has a characteristic in which the low-κ material 311 isburied in the interconnection-to-interconnection gap 306. In this way,though the interconnection-to-interconnection capacitance increases, alevel difference is less likely to be formed in the surface of thesecond insulating film 307. Accordingly, the process step of planarizingthe surface of the deposited second insulating film 307 by CMP is notneeded, thus reducing the number of fabrication process steps. Inaddition, in the third embodiment, it is possible to use a porous low-κfilm, whose application to a conventional semiconductor fabricationprocess has been difficult because planarization is difficult because ofproperties of its material, for example.

In the third embodiment, the cap metal films 304 provided with eaves areformed on the first interconnections 303. Alternatively, as in the firstembodiment, a structure in which recesses are formed on the firstinterconnections 303 and cap metal films 304 are deposited in therecesses may be employed. In such a case, the present invention can beimplemented in the same manner.

Embodiment 4

A semiconductor device and a method for fabricating the device accordingto a fourth embodiment of the present invention will be described withreference to FIGS. 8A through 8E and FIGS. 9A through 9E.

First, as shown in FIG. 8A, a first insulating film 401 is depositedover a semiconductor substrate (not shown) in which a semiconductoractive device is formed, and then recesses to be interconnect trenchesare formed in the first insulating film 401 by lithography and dryetching. Subsequently, a barrier metal film is deposited over the firstinsulating film 401 including the bottoms and walls of the recesses, andthen a metal film made of, for example, a copper film is deposited sothat the recesses are filled therewith. Thereafter, portions of thebarrier metal film and the metal film extending off the recesses in thefirst insulating film 401 are removed by chemical mechanical polishing(CMP), thereby forming first barrier metal films 402 and firstinterconnections 403, respectively.

In this embodiment, the first insulating film 401 is a silicon dioxide(SiO₂) film. Alternatively, any of other insulating materials for use insemiconductor processing, e.g., FSG or a low-κ material, may be used.The first barrier metal films 402 are generally made of tantalum (Ta),tantalum nitride (TaN) or a multilayer structure of these materials.

Next, as shown in FIG. 8B, to prevent diffusion of copper, a linerinsulating film 411 made of SiN or SiC(N) is deposited over the firstinsulating film 401 and the first interconnections 403.

Then, as shown in FIG. 8C, to form an interconnection-to-interconnectiongap, a resist pattern 412 is formed on the liner insulating film 411 bylithography. The resist pattern 412 has an opening pattern with whichonly a portion of the first insulating film 401 located between selectedones of the first interconnections 403 is removed. A feature of theresist pattern 412 is that the edges of the resist pattern 412 match theedges of the selected first interconnections 403. That is, an openingdiameter r1 in the opening pattern of the resist pattern 412 is equal tothe interconnection-to-interconnection space between the firstinterconnections 403.

Then, as shown in FIG. 8D, surface processing is performed on the resistpattern 412, thereby swelling the resist pattern 412. In this case, theresist pattern 412 swells (to be changed into a resist pattern 412 a)such that a region r2 where a portion of the first insulating film 401between selected ones of the first interconnections 403 overlaps withthe resist pattern 412 by about ⅓ of theinterconnection-to-interconnection space. Then, suppose the minimuminterconnection-to-interconnection space between the firstinterconnections 403 is S, a fine opening pattern having an openingdiameter less than or equal to the interconnection-to-interconnectionspace S between interconnections formed at the minimum resolution oflithography is formed as an opening pattern of the resist pattern 412 a.In this manner, even when misalignment occurs between the resist pattern412 a and the first interconnections 403, it is possible to preventexposure of the first interconnections 403 in subsequent process steps.In addition, as shown in FIG. 8C, the edges of the resist pattern 412and the edges of the first interconnections 403 match each other, sothat the resist pattern used for forming the first interconnections 403can also be used as the resist pattern 412. As described above, it issufficient to provide preparation for misalignment by swelling theresist pattern 412 to form an opening pattern finer than that formed atthe minimum resolution of lithography. Accordingly, the edges of theresist pattern 412 and the edges of the first interconnections 403 donot necessarily match each other.

Thereafter, as shown in FIG. 8E, anisotropic etching is performed on theliner insulating film 411 using the resist pattern 412 a formed at theprocess step shown in FIG. 8D as a mask, so that a portion of the linerinsulating film 411 exposed in the opening pattern of the resist pattern412 a is vertically removed to have the upper face of the firstinsulating film 401 partly exposed. When the upper face of the firstinsulating film 401 is exposed in this manner, the etching is stopped.

Then, as shown in FIG. 9A, etching having high selectivity with respectto a portion of the first insulating film 401 located between selectedones of the first interconnections 403 is performed, thereby forming aninterconnection-to-interconnection gap 406. At this time, the linerinsulating film 411 is not etched to remain. Because of this isotropicetching, a recessed portion 406 a at the bottom of theinterconnection-to-interconnection gap 406 is rounded. Instead of theisotropic etching, wet etching may be performed. In this manner, theresist pattern 412 a projects from each of the first interconnections403 to the degree corresponding to the region r2, so that the firstinterconnections 403 are not exposed from the face subjected to etchingeven at the occurrence of misalignment. Accordingly, the surfaces of thefirst interconnections 403 are not damaged. Since etching damage on thesurfaces of the first interconnections 403 is prevented in this way, thestructure of this embodiment is very useful for enhancing the yield andreliability of the first interconnections 403 and, in addition, forpreventing peeling of insulating films formed on the firstinterconnections 403. Furthermore, if the first interconnections 403 aresubjected to etching damage, there arises the problem of contaminationof etching apparatus caused by scattering of copper into the apparatus.In the foregoing process steps, however, the first interconnections 403are free from etching damage, so that the problem of contamination ofetching apparatus does not occur.

Thereafter, as shown in FIG. 9B, a second insulating film 407 isdeposited over the first insulating film 401, the liner insulating film411 and the interconnection-to-interconnection gaps 406, thereby formingan air gap 408 out of the interconnection-to-interconnection gap 406between the first interconnections 403. As a material for the secondinsulating film 407, a material exhibiting poor step coverage ispreferably used because the purpose in using this material is to formthe air gap 408 out of the interconnection-to-interconnection gap 406.In this embodiment, SiO₂ is used for the second insulating film 407.Alternatively, any of other insulating materials for use insemiconductor processing, e.g., FSG or a low-κ material, may be used. Inthe conventional example, an insulating film deposited after formationof the first interconnections 403 is made of SiN or SiC(N) forming aninsulating film preventing diffusion of copper. On the other hand, inthis embodiment, since liner insulating film 411 is formed on thesurfaces of the first interconnections 403, prevention of copperdiffusion does not need to be taken into consideration. Accordingly, amaterial having small capacitance is freely selected as a material forthe second insulating film 407. In addition, a level difference 407 a isformed in a surface portion of the second insulating film 407 locatedabove the air gap 408.

Thereafter, as shown in FIG. 9C, the surface of the second insulatingfilm 407 including the level difference 407 a is planarized by CMP.

Subsequently, as shown in FIG. 9D, lithography and dry etching areperformed to form, in the second insulating film 407, a connecting hole407 b in which one of the first interconnections 403 is exposed andinterconnect trenches 407 c. As in the conventional example, theconnecting hole 407 b and the interconnect trenches 407 c are formed bya dual damascene process.

FIG. 9D shows a state in which the connecting hole 407 b slightly shiftsfrom an associated one of the first interconnections 403 at theoccurrence of misalignment. Even in such a case where misalignmentoccurs, failures such as penetration of the connecting hole 407 bthrough the air gap 408 do not occur at all because no air gap 408 isformed at both sides of the first interconnection 403 to which theconnecting hole 407 b is connected.

Thereafter, as shown in FIG. 9E, a metal film is buried in theconnecting hole 407 b and the interconnect trenches 407 c, and thenportions of the metal film extending off the connecting hole 407 b andthe interconnect trenches 407 c are removed by CMP, thereby forming avia 409 and second interconnections 410.

Now, a mask layout 8A for forming the resist pattern for use in theprocess step shown in FIG. 8C will be specifically described withreference to FIG. 10. FIG. 10 is a schematic plan view for describing anexample of a mask layout of a resist pattern 412 for use in the processstep shown in FIG. 8C. FIG. 8C corresponds to a cross-sectional viewtaken along the line VIIIc-VIIIc shown in FIG. 10.

In FIG. 10, the interconnection width (minimum interconnection width) ofthe first interconnections 403 formed at the minimum resolution oflithography is represented as L and the distance between the firstinterconnections 403 (minimum interconnection-to-interconnection space)formed at the minimum resolution of lithography is represented as S. InFIG. 10, the positions of the connecting holes 407 b formed in asubsequent process step (see, FIG. 9D) are also shown.

As illustrated in FIG. 10, the mask layout 8A of the resist pattern 412includes: a mask region 8 a (provided with dots) covering, with aresist, regions over the connecting holes 407 b and regions over thefirst interconnections 403; and an opening region 8 b (provided with nodots) for exposing the regions between the first interconnections 403except for the mask region 8 a.

The mask layout 8A is formed by automatic design in which a regionhaving an interconnection-to-interconnection space wider than or equalto S and narrower than 3S is automatically detected and an openingregion is formed in the detected region. With respect to the regions forthe connecting holes 407 b, a mask region is formed so as to maskportions each expanded by S+(L/2) from an associated one of the regionswhere the connecting holes 407 b are to be formed, and then this maskregion is subtracted from the previously-formed opening region. In thismanner, the mask layout 8A including the mask region 8 a and the openingregion 8 b for forming the resist pattern shown in FIG. 8C is designed.

The use of the mask layout 8A designed as described above allowsformation of a resist pattern with which an air gap 408 is formed in aninterconnection-to-interconnection space wider than or equal to S andnarrower than 3S and no air gap 408 is formed at both sides of the firstinterconnection 403 connected to the second interconnection 410 throughthe via 409. The reason for detecting the opening region in the rangewider than or equal to S and narrower than 3S and the reason for maskingportions expanded from the formation regions of the connecting holes 407b by S+(L/2) are the same as those described in the first embodiment.

1. A semiconductor device, comprising: a plurality of lowerinterconnections formed at intervals in a first insulating film; asecond insulating film formed over the lower interconnections and thefirst insulating film; a connection portion formed in the secondinsulating film and connected to one of the lower interconnections; andan upper interconnection formed in the second insulating film andconnected to the connection portion, wherein between the lowerinterconnections, a low-dielectric-constant film is formed to fill aninterconnection-to-interconnection gap formed by removing a portion ofthe first insulating film between the lower interconnections, and iscovered with the second insulating film, the connection portion isconnected to one of the lower interconnections not adjacent to thelow-dielectric-constant film, and a part of the second insulating filmcoming into contact with the low-dielectric-constant film and a part ofthe second insulating film formed on one of the lower interconnectionsnot adjacent to the low-dielectric-constant film are the identical film.2. The semiconductor device of claim 1, wherein aninterconnection-to-interconnection space X between the lowerinterconnections sandwiching the low-dielectric-constant film satisfiesthe following relationship:S≦X where S is a minimum interconnection-to-interconnection spacebetween the lower interconnections formed at a minimum resolution oflithography.
 3. The semiconductor device of claim 1, wherein thelow-dielectric-constant film is located, from the connection portion, atleast at a distance y satisfying the following relationship:y=S+(L/2) where S is a minimum interconnection-to-interconnection spacebetween the lower interconnections formed at a minimum resolution oflithography and L is a minimum interconnection width of the lowerinterconnections formed at the minimum resolution of lithography.
 4. Thesemiconductor device of claim 1 wherein a cap layer is formed betweeneach of the surfaces of the lower interconnections and the secondinsulating film.
 5. The semiconductor device of claim 4, wherein the caplayer has a width larger than the interconnection width of the lowerinterconnections and has an eave for an associated one of the lowerinterconnections.
 6. The semiconductor device of claim 4, wherein thecap layer is made of one or more materials selected from the groupconsisting of Ta, TaN, Ti, TiN, W, WCoP, CoB and NiMoP.
 7. The method ofclaim 1, wherein the surfaces of the lower interconnections are lowerthan the surface of the first insulating film.
 8. The semiconductordevice of claim 1, wherein each of the lower interconnections and theupper interconnection is made of a metal mainly containing Cu.
 9. Thesemiconductor device of claim 1, wherein each of the first and secondinsulating films is made of SiO₂, FSG, SiOC or an organic polymer. 10.The semiconductor device of claim 1, wherein the bottom of theinterconnection-to-interconnection gap is located at a position deeperthan the bottom of one of the lower interconnections adjacent to theinterconnection-to-interconnection gap by about ⅓ of the interconnectionwidth of the lower interconnection.